The purpose of the study was to isolate and characterize human ribonucleases in order to more fully understand their physiological role. Since ribonucleases have been found in elevated levels in serum and urine of cancer patients, characterization of the enzymes as to tissue of origin would aid in determining the interrelationship of ribonuclease activity and the disease state. A human liver enzyme was purified 2500-fold using poly G affinity chromatography. The spleen enzyme which was found to be antigenetically similar to the liver enzyme was purified on an immunoabsorbant containing bound antisera to liver enzyme. The enzymes were characterized and found to be very similar in their specificities and response to polynucleotide inhibitors and polyamines.